feat(announcements): Add support for customizable homepage

[GabDug]

Hey, I just made a Pull Request!

This MR integrates with the custom homepage of the @backstage/plugin-home plugin.

This plugin will manage the card and the layout, hence the need to extract the "inner" content of the AnnouncementsCard in another component, so the card logic can be managed by the custom home plugin.

For custom homepage users:

• Added a ready to use HomepageAnnouncements to drop into the <CustomHomepageGrid>
• TODO: Export the inner content of the card, so users can declare their own cards as code for further customization
• TODO: Added tests and documentations with screenshots for the two integrations methods

For plugin maintainers:

• TODO: Updated the dev app

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-announcements	workspaces/announcements/plugins/announcements	minor	v0.5.6